Drilling muds are complex, typically non-Newtonian fluids that serve multiple, critical functions in drilling wells for oil and gas extraction. The fluid is used to remove formation drill cuttings from the wellbore, and the fluid adds hydrostatic mass to help prevent uncontrolled flow of hydrocarbons from the well. The fluid also enables buoyancy to counteract the weight of the drill pipe so that one can drill deeper wells. The fluid also lubricates the bit and stabilizes the wellbore as drilling continues deeper. Limiting the loss of fluid to the recently drilled formation is another important function, and limiting fluid loss typically means the use of bridging agents that are sized particles. It is essential to know the properties of the fluids so they can perform their many functions efficiently.
Some fluid properties are relatively easy to determine in line as the fluid is being used. For example, a Coriolis Meter can accurately determine the flow rate and density of the fluid, but determining rheology of a complex drilling fluid is more complicated; it is commonly done by a manual mud check according to API 13B. Accurate knowledge of a fluid's rheology is required to calculate a Yield Point and plastic viscosity. If the mud is too thick, the mud pump cannot pump it. If the fluid is too thin, it may not suspend the solids that have to be removed from the wellbore as one continues drilling deeper. To continue drilling deeper, drill pipe has to be added to the string. During additions to the string, the mud is no longer being pumped, and Yield Point, which is part of the rheology, determines the pressure needed to move the fluid again after it has been static in the wellbore. If Yield Point (YP) is too high, the pump cannot begin to move the flow of mud.
Prior to the present invention, it has been common not to attempt to shear mix a drilling mud before it is sent down the well to the drill bit, but rather to utilize the drill bit itself to shear mix the drilling mud. This means the rheological properties of the drilling mud are not the most desirable when the mud arrives at the point of drilling, and often can be far from optimum. Moreover, the drilling process adds drill cuttings and other solids and fluids to the drilling fluid, which continuously change significantly the physical properties of the fluid. The prior method, relying on the drill bit for shear mixing, injects considerable uncertainty into the overall process.
One reason the art has relied on the drill bit for shear mixing is that there had not been available a practical way to shear mix the ponderous drilling mud components in a continuous recycling mode.
It takes time to run the “mud checks” specified by the American Petroleum Institute (API) 13B. Mud checks require a skilled operator to successfully run and to report the mud properties. Without shearing the mud, however, the chemistry is not fully activated and the desired rheology is not achieved. In the laboratory either a Hamilton Beach blender or a Silverson mixer is used to imitate the shear developed by a trip through the drilling bit. There is disagreement about which device to use and the amount of time required to mix the mud before running a mud check. Both the Hamilton Beach blender and the Silverson have commercial units that replicate their laboratory units, but they are not typically used for large batches at a drill site for a number of reasons. One problem is simply time. Typically in the laboratory it is common to make 350 ml portions to represent one barrel of fluid. If you shear a one barrel equivalent drilling fluid sample in the laboratory for 5 minutes, then to “scale up” the shear process at the wellsite, it takes the same 5 minutes per actual barrel. Unfortunately it takes too much time. If a rig has 1,000 bbls of drilling fluid, it would take 5,000 minutes or 3.5 days of processing to equal 5 minutes of shear used in the laboratory for the 1 bbl equivalent volume.
Volumes of drilling mud can range from 500 bbls to over 10,000 bbls on location that is stored in pits or tanks, and the mud can stratify based on the density of the additives. When relying on samples for API 13B, it is critical that they are representative of the drilling fluid to be used in the well, but all too often, imperfect sampling practice introduces errors into the API 13B procedure.
Rheology of drilling muds is measured using a Fann 35 or equivalent rotational viscometer that directly reads viscosity on a dial at different rpm. The dial reading is based on the deflection of a bob inside of a rotating cylinder, and the instrument must be calibrated regularly to be accurate. Temperature changes mud rheology, and to determine an accurate downhole rheology means the mud must be heated before measuring its viscosity. By definition, a mud check is done “offline” which takes valuable time and can delay critical decisions about well control. Rig time is often lost while the fluid is circulated in the hole to adjust drilling fluid for the proper rheology after a time delay and before continuing to drill.
A better way to conduct the shear mixing and the rheology measurement process is needed. Ideally a realtime, inline measurement of the mud properties is desired, but there are several challenges to its achievement. One challenge is simply the shear that happens at the bit needs to be replicated at the surface. There are high-pressure mixing devices that accomplish this shear, but they are expensive to build and operate; moreover, high-pressure is an HSE (health, safety and environmental) issue. The rheology measuring device is another challenge. Rheology measurement is used in numerous industries and there are a number of devices adapted for oil field use that include, but are not limited to, the Brookfield TT-100, Grace M3900, and Chandler 3300. The advantage of these types of devices is that they can be calibrated to replicate the Fann 35, and Fann 35 readings have become a de facto standard and it is not uncommon for mud engineers to quote viscosity at various Fann 35 speeds, or add chemistry based on a particular Fann 35 reading. A Fann 35 is a Couette style viscometer as are these three devices, and while they can be correlated to Fann 35 readings, they have intricate internal parts and small flow lines that can easily plug when fluid loss additives are in the drilling fluid. There are numerous other viscometers used in other industries that presumably would also work; however, a viscosity measurement is taken at a single shear rate or at shear rates that are harder to relate to a Fann 35 viscosity reading.
Rheology requires a shear rate vs shear stress curve to accurately calculate plastic viscosity and yield point. A pipe rheometer can be used to measure viscosity by accurately measuring the pressure drop across a known length of pipe of a known internal dimension while measuring an accurate flow rate. Pipe rheometers are commercially available from Chandler Engineering, Stim-Lab, Inc, or Khrone but they are relatively simple devices that can easily be built assuming an understanding of flow and viscosity calculations that are widely published. An example of the calculation required has been published by Petroleum Department of The New Mexico Institute of Mining and Technology as a class exercise available on the Internet as “L5_PipeViscometer.pdf”.
An ideal device to measure flow in a pipe rheometer is a Coriolis meter which has a full opening pipe internal diameter such that it is not easily plugged. A Coriolis meter also gives an accurate mass flow, not just a volume flow rate. Coriolis meters such as the E+H Promass 83I can also measure viscosity. Given the critical performance required of drilling muds to ultimately prevent uncontrolled well events, using a combination of rheology measurement devices based on different principles would make sense. For example, a pipe rheometer requires accurate flow rate. Using the E+H Promass 83I for accurate flow rate could also validate the viscosity being reported by pressure drop. Whereas the pipe rheometer calculations are based on flow and pressure drop, the Promass 83I viscosity is a function of a vibration frequency.
Even with otherwise proper rheology measurement techniques, heat is an additional challenge. The fluid rheology should be measured at more than one temperature. Therefore the ideal device would shear the mud to replicate the shear imparted by the drill bit, heat the fluid to the proper temperature and report rheology at different predetermined temperatures.
Another challenge is where to take a sample. Drilling fluid often stratifies in a tank. A sample taken at the top of the tank, or at any single level, will not be representative of the composition in the entire tank.